Production of sulfur dioxide



Aug. 26, 1952 w. K. LEWIS PRODUCTION OF' SULFUR DIOXIDE Filed Deo 22, l948 lmON.

mzoN. l\ .zorrndown x NT QW M3340 m53 c l n i zOTrmDaEOU ).u/Qzoumm (J2-irren. 14. Lewis Unvenbor- UNITED STATES PATENT o OFFICE Warren" K. Leivis`,Newton, Mass., assigner to Standard Oil Development Company, a corporation of Delaware Application' December 22, 19218, senatore. 66,703

(o1.` ze--m),

7 Claims.

This inventitml is concerned-With theproduction of sulfur dioxide and particularly the production of sulfur dioxide substantially freeof sulfur'trioxide, oxygen, and sulfur` Atithe present'time'sulfur dioxide is produced by the burning of sulfur, HiS, or sulde ores such'asiron pyrites in a sulfur burner employ:- ing air or oxygen as the 'oxidizingyagenti` The sulfunburner isA ordinarily operated under conditionsfemploying'excess' air. The gaseous productl emerging from the burner is therefore a mixtureof SO2, S03. O2, sulfur and nitrogen.

vIn the burning of sulfur to produce SO2 tWo main problems are presented as iollovvs:

A: How tol maintain at allV times the ratio of air to sulfur atthe level `theoretically required for complete combustion of the sulfur exclusively to SO2; and,

Bf The attainment'of complete oxidation of th"s'.`11fito SO2.

Eikenl if "A` were practically possible, oxidation of` thefsulfur would not go completely toSOz sincesome of the oxygen would be consumed Ain conversion of SO2 to S03; some oxygen `would be recovered unchanged,l and some sulfur would accordingly` also appear in' the burner off-gas; Even if an excess of air is used complete oxidation `of sulfur is' not assured.` An analogy exists inthe combustion of coalV in theoperation of -a` steam boiler. The best modernpractice is tc employfabout 15% excess-air; However, even in thevprjesence of such-an excess of air about0.2% to ;3% ofcoalis burned to CO only.` The same problernof incomplete combustion exists in the burning of sulfur;

Itis oftentimes desirable' to produce ar sulfur dioxide gas freefof contaminants ysuchas oxygen, S05; and i sulfur; For example, inthe production of alkali 'or alkaline earth bisultes used in the paper industryQa bisulfit free of sulfur and sulfate is particularlydesirable; It isknown that` theA paper industry* cannot tolerate appreciable amounts of `free sulfurin the SO2 used to prepare bi'sulte liquor.` Therefore excess oxygenin the form off air is employed in the burner' to insure its absenceas" far aslpossible. However, the use of excess air causes excessive N2 dilution oi the S02"C and the" formation of l considerableV amounts of S032"V Under present practice in the production of bisulfi'teliquors for the paper industry,` alf-A kal'in 'earth carbonatos or hydrate's are reacted with an SO2 gas containing oxygen and S03. The latter: impurities when present'in the SO2 `confsinne "excessive Vamounts lof base; resulting` in the "formaton'of" alkaline earth sulfate which bustion zone thereby (1) compensatirigfo de'- iiciencies andA excesses of air furnished to the combustion zone, (2) assuring statistically and onthe averagea supply of the theoreticalamount of air thereto,` and (3) assuring the absence of unburned sulii'ir,` free oxygen and S03 in the final SO2 product stream. it is an object oi this invention therefore to produce vsulfur dioxide free' of sulfur, oxygen and sulfur trioxide. fIt is also an object of this invention to pr'ovide acontinuous method `for preparing` such SQ; bythe burning of sulfur or sulfur-contain#- ingcompounds.

It is another object of this invention` to `prepare sulfites and bisultes free of sulfur,` su'lfates and other oxidation and reactionV products.

These and other objects of the invention will be apparent from the following description of the invention. According to this invention,` pure" SO2f1'ee of oxygen and sulfur contaminantsmay be' continuously preparedby (f1) burning sulfur, hydro-` gen sulde, or sulfur-containing ores, etc., with air and/or oxygen, and (2) passing theresulting gaseous product` containing SO2' contaminated with sulfur and/or oxygen and S03 through a purification zone containing a mixture of FesOi and Fe203, which, acting inthemanner of a fly- Wheel, balances out the positive or negative deviations from thetheoretical in the ratio of air to sulfur supplied to" the sulfur burner, preferably combined with (3)` controlling the amount of air and/or' oXygen fedto `theburner according# to the state'offoxidation of Athematerial inthe purication Zone; The mixtureV of iron oxides employed shouldjpreferably contain notsubf stantially more than weight per cent FeOn; and `10y weight per cent FeaOi; or not substantial-j ly more than 90 weightper cent FeaOjr" a r 1`di 10J weight per cent FezOs. The preferred'- mixture isa 50i-5 0 weight per cent mixture o 'these oxides, Thepresence 'of FezOs isnecessary to `assure the oxidation orany sulfur to, sogwhuewhe prsence ofis `necessary to "absorb any oxygerifor to 'decompose'any S03. The pure oxides or ores rich in the oxides may be employed. Thema-1 terial should be maintained in a dry state and preferably in a finely-divided condition. If a 50-50 Weight per cent mixture of the two oxides is employed, a bed of the powdered mixture four feet deep will exert a satisfactory ywheel action of the type described above on a sulfur burner eiuent containing an excess of as much as l per cent oxygen or sulfur even for as long as one hour to produce a satisfactory product gas.

In order to maintain the sulfur dioxide product from the sulfur burner free of O2, SO2, and sulfur, the amount of air or oxygen fed to the burner is controlled according to the state of oxidation of the material in the purification zone. If too high an oxidation rate of lesO:t to FezOa is occurring in the purification zone, this is an indication of the presence of excessive amounts of Oz and S03, and the amount of air to the burner is cut down. If the FezOa in the purification zone is being too rapidly reduced to FeaOt, it is an indication of the presence of excess sulfur resulting from incomplete combustion, and more air therefore is admitted to the burner.

The state of oxidation of the oxide in the puriication zone may be determined: (l) by sampling and analyzing the gases which have passed through the bed of oxides in the purification zone, (2) by measuring the magnetic strength of the iron oxides present in the bed, (3) by withdrawing a portion of the iron oxides from the purification zone into a separate vessel, blowing preheated air through the withdrawn oxides to -maintain satisfactory reaction and measuring the increase in temperature imparted to the air due to the exothermic heat effect of the oxidation of AFesOi, or, (4) by measuring the increase in temperature of the gas after passing through the main bed of oxides in the purification zone itself. v

Automatic controls either electric, magnetic or otherwise may be set up to govern the admission of air to the sulfur burner depending upon the state of oxidation of the iron oxide in the purification zone.

Reactions occurring in the purification zone are as follows:

This reaction is extremely rapid even at temperatures as low as 7 50 C.

This 'reaction is rapid at temperatures in the neighborhood of 750 C. or above and proceeds smoothly even at lower temperatures.

This reaction is rapid and quantitative. Neither S03 nor sulfur is formed. The reaction starts at 750 C., goes well at 800 C., and very fast at 900 C. This reaction is catalyzed by SO2. If a reaction vessel containing a mixture of FezOa and FeS is fluidized with SO2, the reaction goes approximately thirty times as fast as with nitrogen at a temperature ofn 800 C. However, at 900 C. the reaction is very rapid even when fluidized with nitrogen.

Both reactions 4 and 5 are undesired but do occur locally at specific points in the bed. vThe bed must be deep enough so that reaction 5 eliminates FeS from reaction 4. Fortunatelgs the presence of SO2 very greatly accelerates reaction 5.

The equations demonstrate that when excess sulfur is present the condition is remedied by reactions according to equations l, 4 and 5. When excess oxygen or S03 are present, the situation is corrected by reactions according to equations 2 and 3.

FeS and FesOr are relatively unreactive in the presence of each other and reaction apparently does not take place even at 900 C.

Unfortunately, FeS reacts with SO2 at the temperatures required for this process to form FesOr and elementary sulfur vapor. The reaction goes only to a small extent, but in the absence of FezOz elementary sulfur vapor cannot be completely removed from the gas. Appreciable formation of FeS in the bed should be avoided, which is equivalent to saying that the reduction of the iron oxide should not go below the FesO4 stage.

While it is seen therefore that at one time air is supplied to the burner in excess of that required to convert the sulfur to SO2, and at other times air is supplied in amounts insuflicient to convert the sulfur to SO2, nevertheless, it will be understood from the process outlined that, statistically, the amount of oxygen fed to the system overall is on the average the theoretical amount required to convert the sulfur to SO2.

The manner in which the present process is carried out will be fully understood from the following description when read with reference to the accompanying drawing which is a semidiagrammatic view in sectional elevation of one type of apparatus suitable for the purpose.

Referring to the drawing, numeral I represents a sulfur burner in which a gas rich in SO2 is pro-duced. In the burner the sulfur dioxide may be made from iron pyrites, but other sources such as sulfur, hydrogen sulfide, spent oxide from gas works, and flue gases produced in ore Smelting may be employed. The type of burner used varies with the material being burnt. Ordinary commercial burners may be employed. Finely divided materials demand the use ofr special furnaces fed with stirrers, the purpose of which is to constantly expose fresh surfaces of the burning material to the air or oxidizing agent. In this regard, numeral 2 represents a source of supply of sulfur fed to the burner While air or other gas containing oxygen is added to the burner via line 3. The gaseous materials may pass fromthe burner via line 4 to a secondary clean-up zone 5 in which any unburned sulfur is removed, etc., but the necessity of such a zone is eliminated or greatly reduced by the process of this invention. The hot gaseous SO2 containing oxygen, sulfur, and S03, among other impurities, is removed via line 6. Since prior to this invention the burner has been ordinarily operated with excess air (e. g. 15 excess) in order to accomplish the most complete removal of sulfur, excess oxygen and S03 have always appeared in the SO2 stream. In the practice of this invention an excess of air is fed to the burner alternately with a deciency of air to average out to the theoretical, the flywheel action of the purification zone supplying the necessary balance. The hot SO2 gaseous streamv leaves the burner at approximately 800 C., and,.

if desired the temperature may be adjusted in heat exchanger Il. Thehot gaseous streamis then led into purication zone 1. Purification zone I contains a bed B of oxides of iron, preierably a 50-50 weight per cent mixture of FezOa and FeaOi. The oxides of iron may consistof the pure oxides, ores rich in the respective iron oxides,

Y 5` `fIt issoundjpractcetofhave the iron oxides 4mixture contain]between l0% and 90% by :weight ofjEezO and between 90% and'10%7` by weight `of l'fesOi'.4 AIt is preferable to maintain the iron oxides at 'a U50-50 weight Vper centmixture. 'The-tem- `perature maintained in the bed lies between 700 Yand `IO00 C., prefer`ably 850 to900" C.` The thicknessi" of theibed depends" upon `operating condi'- ttio A` thinv bed may be employed whereby the limitthereof is uniform gas-solids contact withlfoutjchanneling.` Athickbedmay be employed, butthe height of the bed is limited by the increase in pressure drop. `The hot gaseous SO2 containing the impuritiesdescribed, is allowed to 4enter the bottom of the purication zonejj'l at sucharate as will produce the so-c'alled duidizing effect in zone "l, that "is,l will makeiconditions `uniform 'throughout the bedf'lhe `jbed isV fluid- 'ized'fin order to avoid channeling and overheatin ofthe bed at the bottom thereof.` The presr sii'ieor'i the bed may be substantiallythat em- :ployedfon tlieburner. In the bed any oxygen or S703 reactsat a rapid 'rate with the lower iron oxide, Fe3O4, according to the equations previous- -ly outlined. Constant changes will occur in the f'* .composition of the iron oxide duringfthe passage "ofthe gas depending upon the suirur, oxygen, and 4$05; content of the gas. `As previously explained, it `inayhbe possible 'for l the entire bed to be con- Mvderted'ondthe onev extreme entirely `to lFeiOe and onsthe other extreme to` Fezos. However, `the stateofi the `bed is preferab1y controlled-to Aricain- Htain at all times an iron oxide orV mixture of iron .Odesat least 105%, by weight of wmenis ma state of" oxidation belowFegOa, and .at least"l0% t by weight of which is in a state of oxidation above FeaOi. The amount of air fed to the combustion zone is controlled, therefore, according to the Y state of oxidation determinedin thebed. Statistically, the amount of'oxygen fed to the system is. on the average-,the theoretical amount required to convert the sulfur to SO2. Theappear- ,anceuof too great a quantity of FezOs inthe, bed is indicative of excessive oxidation of the'iron oxide `due to Oxygen and` S03 contained in the `gaseous stream entering the purication zone; therefore, the amount of air `being supplied to the .burner is decreased in the manner outlined above iso `as'ito compensate for the excess bya corre- *spending air deficiency and to balance `the aver- A that` will substantially pass through 100 mesh screen sdesrable. However, material of200 to L flQOmmesh may'alsobe employed. `Within theA purification ,-zone, the oxide bed is maintained in a state of turbulence by means 0f the iluidizing action of the gas entering the bed. vIn this manin e'rl localized overheating is avoided and uniroijm conditions are produced throughout the bed. 7;, Sulfur dioxide, free of oxygen, sulfurand S03.

"is removed overhead from the purification zone nvia line ll), after which it is cooled and stored. lllitrogen is the chief contaminant of the puried S02- f v Y "As Apreviously related, the state of oxidation onces-iii meriti-irisation bedr The side Astream withdrawal *isf an lexceuei'ptfinethod, f costruito keep the desired ratiofofsFez'Oa tojF'esQr in1 the `main unit. If desired, the vessel'employed in 1the `airiblowing 'of this side stream l`can berr. "'larged in'siz'e, still serving the lsame purpose, but 4containing a considerable -reservoirjjbf:oxide substantially fully oxidized to FezOiq` Tlieipresinthe event "that, due jtfo in'advertericefthe oxygenjcontent of the 1oxidesjin the main bed in the purification "ione `bvedzimes so small that sulfur vapor" appearsdn` "the out-let" g'a's. he

jre'serye bed b f F8203 yin the auxiliary "ves'su aan be employed -for almost instantaneous correction of this situation. `Asstateci boveifA theoxygen content becomes too high fand "oxygen, "appears in 'the outlet gasffrom the *summation zone.

one fanedtiowaq the oxygen suppiyito me miur burner toa point wherejit is Vassuredly "insuiiilcientj and the situation will correct itself i` Ao alternative forlassuring the oxidation-coif- A"trol is that itis possibleto Aoperate heburner up theoxygen4 deficiency. i s i "At "atmospheric pressurefapproximately `sq. `itluf cross section in the f luidized lpuriiication unit are'reguired for each `ton ofrzsulfurburne'd withfair per dayr` 100 lbs. yof'505 0 vmixture' of powdered Fe3O4 "and Fe'zOsper sq. ftlwillrnaintain the required purication for one hour in the purification zone in the presencefo 10% `excess sulfur or oxygen. To `maintfain 1thisnirification, only 3 toV 5 lft.` depth of bed, dense phase, is required. Thebed deptht'o `obtain satisfactory control is so shallow that vparticular atterrtion should be paidto *insure uniformpressre drop across Vthe grid, l andthereby maintain uniform distribution. The pressure f'drop `tliroiigh the grid should be a considerable fraction of that `through the bed itself; i The pressure fdrop Vthroughthe bedcan be `less than 1.5 :inches of mercury.

-In` the event that the S02 is to ybe vi-inplciyed for the-production of bisulfite liquor for usein the paper industry', the gaseous SO2 `may lbe 4withdrawn via line H and bubbled into a solution of calcium carbonate or a slurry ofllime entering absorption "zone l2 via line I3. The bisulte liquor, free 'of sulfate,-is removedfrom absorption zone I2 viajline I4.

During `the, purification of the SO2 `stream in the vessel 1, it may be desirableto add amounts `of oxygen to the gaseous `stream-in"additioni'to ythat introduced into the `Vstream via-the `com2- ture level ofthe bed. That point might also be employed for the introduction of sulfur vapor in the event that the oxidation gets out of "control .due to the presence of .excessive oxidizing agent.

However, under proper operating conditions the behavior of the iron oxide bed itself is used A"eiii'ciently 'to control the amount of air entering the sulfur burner. When the pure SO2 stream is employed to prepare bisulte liquor, considerable having in calcium or other base occurs. 'I'he product is free of sulfate which would normally -consume additional amounts of base material.

and which is likewise undesirable in the bisulte liquor. The presence of S'Oa normally found in the gaseous SO2 likewise produces corrosion problems which are hereby avoided. A. It will be understood that the process outlined is but one modification of the invention, and

numerous modifications will occur to those skilled inthe art Without departing from the spirit of the invention.

For example, solid systems other than FezOs- VFeaOi may be employed. However, it is necesthat reduction of S03 or O2 by the rst solid and oxidation of S to SO2 by the second solid are both substantially quantitative. Other suitable Vsystems are copper oxide-copper; MnO and its air oxidation products at the temperature level .in question;v certain spinels; molten salts, etc.

Mixtures vof agents thus described function satisactorily. The copper oxide-copper system is not lpreferred since it requires a fluidizingcarrier such as silica gel. Also, during a period of insufcient air supply, it tends toy accumulate sulful`in`the bed, Atherebyrconverting the copper to copper sulfide. Due to the presence of the carrier a substantially higher pressure drop is required for the same protective capacity.

i Itv should .be noted that even the system FeaOi-FeS may be employed if vone can tolerate an SO2 gascontaining about 1%. to 2% of sulfur vapor. However, thesystem does not quite possess the necessary oxidation potentials to oxidize the sulfurvsufciently Aquantitatively to produce an SO2 stream entirely free of sulfur. f

VWhile separate zones have been described for the SO2 production and purification, certain advantages may be achieved by employing the purication zone simultaneously as a sulfur burner by supplying sulfur and oxygen to the purication zone.

Having outlined the invention in a manner that it may be carried out byA those skilled in the art, what is claimed as new and useful is:

1. A process for the continuous production of sulfur dioxide substantially free of sulfur, oxygen, and sulfur trioxide, whichv comprises continuously burning sulfur in a combustion zone with an oxygen-containing gas, whereby a gaseous mixture of sulfur dioxide containing at least one impurity selected from the group consisting of sulfur, oxygen, and sulfur trioxide is produced, passing the impure 4gaseous sulfur diloxide into a purication zone containing finelydivided iron oxide at least 10 per cent by weight of which is in a lstate of oxidation of FezOa, and at vleast 10 per cent'by weight of which is in a state of oxidation of FesO-4, maintaining a temperature of 700 C. to 1000 C. in the purication zone, controlling the amount of an oxygen-containing gas fed to the combustion zone to maintain the iron oxide in the purication zone in said required stateY of oxidation and recovering from the puriiication zone gaseous sulfur` dioxide substantially free of sulfur, oxygen, and sulfur trioxide.

2. A process according to claim l in Which the iron oxide bed is a bed containing 50 weight per cent FezOa and 50 weight per cent FeaOi, and in which the oxygen-containing gas is supplied to the combustion zone in the form of air.

3. A process for the continuous production of sulfur dioxide substantially free of sulfur, oxygen. and sulfur trioxide, which comprises burning hydrogen sulde in a combustion zone with an oxygen-containing gas, whereby a gaseous mixture of sulfur dioxide containing at least one impurity selected from the group consisting of sulfur, hydrogen sulde, oxygen, and sulfur trioxide is produced, passing the impure gaseous sulfur dioxideinto a purification zone containing finely-divided iron oxide, at least 10 per cent by weight ofY which is in a state of oxidation below FezOs, and at least 10 per cent by weight of which is in a state of oxidation above Fe304, maintaining a temperature of 700 C. to 1000 C. in the purification zone, controlling the amount of an oxygen-containing gas fed to the combustion Zone to maintain the iron oxide in the purification zone in said required state of oxidation and recovering from the purification zone gaseous sulfur dioxide substantially free of sulfur, hydrogen sulfide, oxygen, and sulfur trioxide.

4. A process according to claim 3 in whichthe ironvoxide bed is a Ibed containing 50` weight per cent FezOs and 50 weight per cent-Fe3O4, and in which the oxygen-containing gas is supplied to the combustion zone in the form of air.

5. AA process for the continuous production of sulfur dioxide substantially free of sulfur, oxygen, and sulfur trioxide, which comprises continuously burning iron pyrites in a combustion zone with an oxygen-containing gas, whereby a gaseous mixture of sulfur dioxide containing at least one impurity selected from the'g'roup consisting of sulfur, oxygen, and sulfur trioxide is produced, passing the .impure gaseous sulfur dioxide into a purification zone containing nely- .divided iron oxide at least 10 per cent by weight of which is in a state of oxidation below FezOs, and at least 10 per cent by weight of which is in a stateof oxidation above FesOi, maintaining a temperature of 700 C. to 1000" C. in the purification zone, controlling the amount of an oxygencontaining gas fed to the combustion zone to maintain the iron oxide in said purication zone in the required state of oxidation and recovering from the purication zone gaseous sulfur dioxide substantially free of sulfur, oxygen, and sulfur trioxide.

l 6. A process according to claim 5 in which the bed of iron oxide is a bed containing 50 weight per cent FezOa and 50 weight per cent Fe304, and

`in which the oxygen-containing gas is supplied containing material in a combustion zone with an oxygen-containing gas whereby a gaseous mixture of sulfur dioxide containing at least one impurity selected from the group consisting of sulfur, oxygen, and sulfur trioxide is produced, passing the impure gaseous sulfur dioxide into a purification zone containing finely divided iron oxide, at least 10% by weight of which is in a state of oxidation of FezOs, and at least 10% by Weight of which is in the state of oxidation of Fe304, maintaining a temperature of 700 C. to 1000 C. in the purication zone, controlling the amount of oxygen-containing gas fed to the combustion zone to maintain the iron oxide in the purification zone in said required state of oxidation and recovering from the purification zone gaseous sulfur dioxide substantially free of sulfur, oxygen, and sulfur trioxide.

WARREN K. LEWIS.

10 REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,018,479 Burkheiser Feb. 27, 1912 1,412,452 Coolbaugh Apr. 11, 1922 1,678,630 Bahr July 31, 1928 2,245,697 Melendy June 17, 1941 FOREIGN PATENTS Number Country Date 4,262 Great Britain Dec. 2l, 1888 10,556 Great Britain Aug, 21, 1916 16,981 Great Britain Dec. 4, 1916 OTHER REFERENCES Mellor, Comprehensive Treatise on Inorganic Theoretical Chemistry (1934) vol. 13, pages 758 and 810. 

1. A PROCESS FOR THE CONTINUOUS PRODUCTION OF SULFUR DIOXIDE SUBSTANTIALLY FREE OF SULFUR, OXYGEN, AND SULFURE TRIOXIDE, WHICH COMPRISES CONTINUOUSLY BURNING SULFUR IN A COMBUSTION ZONE WITH AN OXYGEN-CONTAINING GAS, WHEREBY A GASEOUS MIXTURE OF SULFUR DIOXIDE CONTAINING AT LEAST ONE IMPURITY SELECTED FROM THE GROUP CONSISTING OF SULFUR, OXYGEN, AND SULFUR TRIOXIDE IS PRODUCTED, PASSING THE IMPURE GASEOUS SULFUR DIOXIDE INTO A PURIFICATION ZONE CONTAINING FINELYDIVIDED IRON OXIDE AT LEAST 10 PER CENT BY WEIGHT OF WHICH IS IN A STATE OF OXIDATION OF FE2O3, AND AT LEAST 10 PER CENT BY WEIGHT OF WHICH IS IN A STATE OF OXIDATION OF FE3O4, MAINTAINING A TEMPERATURE OF 700* C. TO 1000* C. IN THE PURIFICATION ZONE, CONTROLLING THE AMOUNT OF AN OXYGEN-CONTAINING GAS FED TO THE COMBUSTION ZONE TO MAINTAIN THE IRON OXIDE IN THE PURIFICATION ZONE IN SAID REQUIRED STATE OF OXIDATION AND RECOVERING FROM THE PURIFICATION ZONE GASEOUS SULFUR DIOXIDE SUBSTANTIALLY FREE OF SULFUR, OXYGEN, AND SULFUR TRIOXIDE. 